1. Field of the Invention
The present invention relates to a surface light source device, a liquid crystal display employing the surface light source device and a light control sheet which is advantageously applied to them. The present invention is applied, for example, to displays for personal computer or navigation system.
2. Related Art
Referring to FIG. 16, shown is an outlined arrangement of a liquid crystal display 29 which is employed, for example, in a portable personal computer. The liquid crystal display 29 comprises a liquid crystal display panel 32 and a surface light source device 33 disposed on a lower side (i.e. back side) of the panel. The illustrated surface light source device 33 is provided with a fluorescent lamp 36, light guide plate 34 and prism sheet (light control sheet) 35.
According to a well-known operation, the fluorescent lamp 36 emits illumination light (primary light), which is deflected by the light guide 34 to be converted into a flux having an enlarged cross section. The flux is supplied to the liquid crystal display panel 32 via the prism sheet (light control sheet) 35.
The liquid crystal display panel 32 comprises a polarization plate 37 disposed on a light input side, another polarization plate (analyser) 40 disposed on a light output side and a liquid crystal display cell 38 interposed between them. Accordingly, the liquid crystal display cell 38 is supplied with the output illumination light of the surface light source device 33 via the polarization plate 37. As known well, the liquid crystal display cell 38 controls polarization state of the inputted light depending on position according to output signals of a drive circuit (not shown).
Then, output light of the liquid crystal display cell 38 transmits through the polarization plate (analyser) 40 depending on state of polarization. After all, light H intensity of which is controlled depending on position is emitted through the polarization plate (analyser) 40. Some of the light H is incident to eyes 41 of an operator of the personal computer, causing the operator to see an image.
The conventional liquid crystal display 29 is, however, subject to a problem that has been unrecognized. That is, screen brightness often varies depending on combination of the liquid crystal display panel 32 and the surface light source device 33 arranged for the device. It is noted that difference in screen brightness arises depending on neither performance of the surface light source device 33 itself nor that of liquid crystal display panel 32, but depending on congeniality between the device 33 and panel 22. Such phenomenon has been unknown and found newly.
Researches tells that the phenomenon occurs in relation to direction of transmission axis of the polarization plate 37 disposed on an input side of the liquid crystal display cell 38. In general, a polarization plate 37 is arranged in a liquid crystal display so that the transmission axis 42 is orientated as shown in either FIG. 17a or FIG. 17b. In FIGS. 17a and 17b, reference symbol E represents an extending direction of prismatic grooves 35a of a prism sheet 35 (cf: FIG. 16), the direction being parallel with that of the fluorescent lamp 36.
It has been found that a great difference in screen brightness arises between a case as shown in FIG. 17a (Case 1) and another case as shown in FIG. 17b (Case 2) under employment of the same prism sheet 35 in both cases.
Therefore, the problem seems to solved passively by means of excluding items (liquid crystal displays) having “bad congeniality” through brightness checking of every individual item, or by means of changing combination so that “good congeniality” is realized. Such ways leads, however, to a reduced efficiency of working. Thus, a positive means to solve the problem has been awaited.